Today printed wiring boards, circuit boards and other elements have smaller wettable surfaces to be solder coated and joined. In co-pending application Ser. No. 07/860,316 is disclosed gas shrouded wave soldering wherein a solder wave has a cover means with a slot for the solder wave to project therethrough and wherein an inert gas supply, preferably nitrogen, is provided underneath the cover so that solder coating of wettable surfaces occurs to circuit boards and the like when they are substantially blanketed by the inert gas.
In co-pending application Ser. No. 07/860,316 reference is made to soldering without the use of flux. Whereas flux may not be necessary in certain cases, there are situations where a fluxing step is advantageous depending upon the type of solder, the size and location of the wettable surfaces to be soldered and the speed or time which the solder is applied. Whereas dross in some instances may form on wettable surfaces after soldering, if one could remove dross and oxide particles from the solder, then some of the problems that have occurred in the past with utilizing flux are avoided. A fluxing step before a preheating step in air cleans the wettable metallized surfaces to be solder coated and joined.
In the aforesaid co-pending application soldering occurs under an inert gas atmosphere (the term inert is applied to gas which includes nitrogen) which prevents oxides forming on the liquid solder surfaces. U.S. Pat. No. 5,044,542 shows shield gas wave soldering wherein shield gas or reducing gas blankets the solder wave during the soldering step. The benefits of soldering in an atmosphere which has a reduced oxygen content are known.
Tardoskegyi in U.S. Pat. No. 3,705,457 and Elliott et al in U.S. Pat. application Ser. No. 549,603 both disclose the use of a gas knife jet. U.S. Pat. Nos. 4,402,448, 4,679,720 and 4,995,411 all assigned to Hollis Automation Inc. disclose the uses of gas or air knife jets for cleaning surface mounted conductor elements carried by printed circuit boards.
One of the problems with flux in the past is that the flux itself caused a deposit or residue on the solder coated surfaces after solder coating and this residue generally had to be removed by cleaning after soldering. However, flux technology has improved and it is now possible to use what is sometimes referred to as a "no-clean" flux which does not leave a residue and does not require solder coated surfaces to be cleaned after solder coating. The term "no-clean" flux is applied to a flux which leaves a low level of residue on solder, the residue generally being non-corrosive and non-conductive. One example of a no-clean flux contains little or no halide, another example is a non-corrosive and non-conductive organic acid dissolved in an ethanol or isopropanol solvent. A further example is common RMA flux which is a mixture of a rosin, such as abietic acid, an activator, such as divethylamine hydrochloride and solvent such as alcohol.
The addition of a small quantity of adipic acid with a solvent such as ethyl or isopropyl alcohol results in a no-clean flux. Furthermore, it is known that low dross solder, which contains from about 10 to 1000 ppm phosphorous, reduces dross formation in a solder pot exposed to air. By utilizing a low dross solder it has been found that the shield gas used to blanket a solder wave during solder coating does not require all oxygen to be removed from the atmosphere. A satisfactory solder coating can be obtained with low dross solder when an oxygen content of about 5% is included in the shield gas. In some situations oxygen contents greater than 5% are satisfactory. Thus the cost of a shield gas is greatly reduced, and this results in savings in the soldering process.